HomeMy WebLinkAboutGILLESPIE FARM - ODP - 29-00 - REPORTS - DRAINAGE REPORTEXHIBIT "B": SUMMARY OF DETENTION POND ANALYSIS AND DESIGN
DETENTION POND 100 YR. STORM Q=CCfIA= 77.0625 I
C=0.45
A=137 Ac.
C,=-1.25
Tc
(min)
100-YR
RAINFALL
INTENSITY
(inlhr)
DEVELOPED
SITE
RUNOFF-
(cfs)
VOLUME
INTO
POND
(cf)
100-YR
MAX. RELEASE
RATE
(cfs)
VOLUME
RELEASE
(cf)
REQUIRE
DETENTIO.
VOLUMEr..a
10
7.7
594.9
356,954
85.6
51,378
7.0
15
6.5
502.4
452,203
85.6
77,067
8.6
20
5.6
431.6
517,860
85.6
102,756
9.5
30
4.5
348.3
626,981
85.6
154,134
10.9
40
3.7
288.2
691,713
85.6
205,512
50
3.2
248.9
746.736
85.6
256,890
11.2;
60
2.9
220.4
793,436
85.6
308,268
11.1
70
2.4
185.0
776,790
85.6
359,646
9.6
80
2.2
169.5
813,780
85.6
411,024
9.2
90
1.9
146.4
790,661
85.6
462,402
7.5
100
1.8
138.7
832,275
85.6
513,780
7.3
110
1.6
123.3
813,780
85.6
565,158
5.7
120
1.5
115.6
832.275
85.6
616,536
5.0
TST Inc.
Consulting 8/24/00
Engineers hydro
TABLE 2. DEVELOPED CONDITIONS HYDROLOGIC CALCULATIONS WORKSHEET
BASIN
DESIGN
POINT
GUTTER "
FLOW
VELOCITY
(fps)
GUTTER
TRAVEL TIME
Tt
(min)
TIME OF CONCENTRATION
2-yr 100 yr
Tc Tc
(min) (min)
RAINFALL INTENSITY
I I
2-yr 100-yr
(in/hr) (in/hr)-
PEAK DISCHARGE
Q Q
2-yr 100-yr
(cfs) (cfs)
A
1
1.50
16.67
36.69
33.22
1.20
4.40
9.72
44.55
B
2
1.50
13.33
33.35
29.89
1.20
4.50
19.98
93.66
C
3
1.50
11.11
26.62
23.93
1.40
5.10
5.04
22.95
D
4
1.50
13.33
27.49
25.04
1.40
5.00
4.41
19.69
E
5
1 1.50
2.22
16.38
13.93
1.80
6.60
9.72
44.55
G
6
1.50
16.67
36.69
33.22
1.20
4.40
17.82
81.68
H
7
1.50
16.67
36.69
33.22
1 1.20
4.40
11.88
54.45
Sum
1.50
22.22
36.69
33.22
1.20
4.40
TST, Inc.
Consulting 8/24,100
Engineers hydromas
TABLE 2. DEVELOPED CONDITIONS HYDROLOGIC CALCULATIONS WORKSHEET
BASIN
DESIGN
POINT
AREA
(Ac.)
RUNOFF
COEFF.
C
FREQUENCY COEFF.
Cf Cf
2-yr 100-yr
OVERLAND
FLOW
LENGTH
(R)
AVERAGE
OVERLAND
SLOPE
(%)
OVERLAND TRAVEL TIME
2-yr 100-yr
Ti Ti
(min) (min)
GUTTER
FLOW
LENGTH
(ft)
AVERAGE
GUTTER
SLOPE
(%)'
A
1
18.0
0.45
1.00
1.25
500
2.25
20.02
16.55
1500
0.60
B
2
37.0
0.45
1.00
1.25
500
2.25
20.02
16.55
1200
0.60
C
3
8.0
0.45
1.00
1.25
300
2.25
15.51
12.82
1000
0.60
D
4
7.0
0.45
1.00
1.25
250
2.25
14.16
11.71
1200
0.60
E
5
12.0
0.45
1.00
1.25
250
2.25
14.16
11.71
200
0.60
G
6
33.0
0.45
1.00
1.25
500
2.25
20.02
16.55
1500
0.60
H
7
22.0
0.45
1.00
1.25
500
2.25
20.02
16.55
1500
0.60
Sum
137.0
0.45
1.00
1.25
500
2.25
20.02
16.55
2000
0.60
. Gutter Flog- Length is the remaining length of a basin that is over 500 feet.
TST, Inc.
Consulting 9/24/00
Engineers hydronnas
TABLE 1. HISTORIC CONDITIONS HYDROLOGIC CALCULATIONS WORKSHEET
BASIN
DESIGN
POINT
GUTTER
FLOW
VELOCITY
(fps)
GUTTER
TRAVELTIME
Tt
(min)
TIME OF CONCENTRATION
2-yr
Tc
(min)
100-yr
Tc
(min)
=RAINFALL INTENSITY
I I
2-yr 100-yr
(in/hr) (in/hr)
PEAK DISCHARGE'
Q Q
2-yr 100-yr
(cfs) (Cfs)
HAI
1
0.70
47.62
76.42
74.82
0.70
250
19.18
85.63
TST, Inc.
Consulting 8/24/00
Engineers hydrornas
TABLE 1. HISTORIC CONDITIONS HYDROLOGIC CALCULATIONS WORKSHEET
BASIN
DESIGN
POINT
AREA
(Ac.)
RUNOFF
COEFF.
C
FREQUENCY COEFF.
Cf Cf
2-N r 100-yr
OVERLAND
FLOW
LENGTH
(ft)
AVERAGE
OVERLAND
SLOPE
(%)
OVERLAND TRAVEL TIME
2-}r 100-yr
Ti Ti
(min) (min)
GUTTER
FLOW
LENGTH
(ft) *
AVERAGE
GUTTER
SLOPE
(%)
HA1
1
137.0
0.20
1.00 1.25
500
2.3
28.80
27.20
2000
2.3
* Gutter Flow Length is the remaining length of a basin that is oN er 500 feet.
TST,Inc.
Consulting 8/24/00
Engineers ]Mromas
TST, INC.
Mr. Basil Hamden
August 30, 2000
Page 3
and adjacent property owners and will work closely to address their concerns of any additional
runoff flowing into their property. However, if the neighbors or the ditch companies are not
willing to reach an agreement to this affect, the City of Fort Collins has suggested that they will
be satisfied with an on -site facility that retains twice the 100 year historic conditions.
The open space tracts will be reseeded with native grasses of the dry land variety. All of the
roadways within this development will be classified as collector or local streets and will be
constructed as called out for in the City of Fort Collins Street Design manual.
Historic and developed runoff conditions were calculated for this site. The development of the
proposed Gillespie Farm will increase peak stormwater runoff flows from historic flows. We
propose to detain on site. Storm water will eventually be routed to the Number 8 outlet ditch.
The detention pond will be sized to restrict runoff to 100-yr historic rates. Please see the attached
spreadsheets for further clarification.
We look forward to your review and comments and will gladly answer any questions you may
have.
Sincerely,
TST INC., CONSULTING ENGINEERS
I 1��1
yvG
Fraser Walsh
FW/cros
Enclosures
ly I
David B. Lindsay, P.E.
TST, INC.
Mr. Basil Hamden
August 30, 2000
Page 2
May 1984. Developed condition storm facilities were evaluated based on the 2-year and 100-
year storm frequencies as dictated by Section 3 of the SDDC manual.
The Rational Method was selected to calculate runoff for the site. The Rational Method utilizes
the SDDC manual equation:
Q = C,CIA
where Q is the flow in cfs, A is the total area of the basin in acres, Cf is the storm frequency
adjustment factor, C is the runoff coefficient, and I is the rainfall intensity in inches per hour.
The storm frequency adjustment factor was selected from Table 3-4 of the SDDC manual based
on the storm return periods. The runoff coefficient, C, was conservatively selected from Table
3-2 of the SDDC manual based on the developed condition land use. The appropriate rainfall
intensity was taken from the SDDC rainfall intensity duration curve. To obtain the rainfall
intensity, the time of concentration was determined by the following equation:
t'=tov+t
where tc is the time of concentration in minutes, t,V is the initial or overland flow time in minutes,
and t, is the travel time in the ditch, channel, or gutter in minutes. The initial or overland flow
time was calculated with SDDC equation from page 3-5:
t; [l.8%(l.l-CCf)D0.51/(s)0.33
where D is the length of overland flow in feet (limited to a maximum of 500 feet), and S is the
average basin slope in percent. This procedure for computing time of concentration allows for
overland flow as well as travel time for runoff collected in roadside ditches or other channels.
The proposed drainage plan consists of a combination of overland flow across lots and open
spaces, collected in curb and gutter, and diverted to low points. Subbasins were delineated based
on the proposed zoning areas.
The Final Drainage Report shall include a more detailed analysis of the proposed road sections
and culvert design, as well as erosion protection and stability measures. Channels and culverts
will be sized to intercept and convey runoff from the 100-year storm.
At this time we are proposing to create a detention pond on the southeast side of the project. The
detention pond will empty into the Number 8 outlet of the Eaton ditch. The proposed outlet will
be constructed on property not owned by Sollenberger Development Corp., SDC, the owners of
Gillespie Farm. The final size and alignment of the pond and the outlets will be determined as
we proceed into the final design of this project. We will contact the appropriate ditch companies
August 30, 2000
Mr. Basil Hamden
City of Fort Collins
235 Mathews Street
P.O. Box 580
Fort Collins, CO 80522-0580
Re: Gillespie Farm
Project no. 953-001
Dear Mr. Hamden:
We are pleased to present the results of an Overall Drainage Evaluation for the Proposed
Gillespie Farm Project. In accordance with the requirements of the City of Fort Collins, the
purpose of this report is to present a conceptual plan for handling drainage on site prior to the
actual sizing of facilities. An overall hydrologic analysis was conducted to review the impact of
the proposed development on offsite drainage and runoff control facilities. The hydraulic design
and sizing for specific control structures will be detailed in the Final Drainage Report.
The Gillespie Farm project is a proposed single and multi -family residential site located in the
Northwest Quarter of Section 32, Township 8 North, Range 68 West, in Larimer County,
Colorado. The site is roughly bounded on the North by Larimer County Road 52 and on the West
by Larimer County Road 11. The south and westerly sides of the proposed development are
undeveloped agricultural ground.
Gillespie Farm consists of approximately 137 acres. The development will have approximately
666 units of single and multi -family residences. The project is located in the Cooper Slough
Master Drainage Basin.
Stormwater runoff across the site consists of sheet flow to the east at an average slope of 2.25%.
There are no offsite runoff contributions to the site. The highest point on the site is located on the
northerly portion of this site. On -site soils can generally be described as sandy clay. The project
site is currently tilled and irrigated farmland.
The majority of the runoff collects offsite in the Number 8 outlet of the Eaton ditch. The number
8 outlet is owned by the Windsor Reservoir Canal Company and the Eaton ditch is owned by the
Larimer and Weld Canal Company.
The drainage system presented in this report has been developed in accordance with the
guidelines established by the City of Fort Collins Storm Drainage Criteria Manual (SDDC) dated
TST, INC. 748 Whalers Way - Building D
Consulting Engineers Fort Collins, CO 80525
(970) 226-0557
Metro (303) 595-9103
Fax (970) 226-0204
Email info@tstinc.com
www.tstinc.com